I have read/watched a few stories like Juni Taisen (No, not Jujutsu Kaisen) zodiac war, and it made me wonder to have no protagonist and instead of multiple sides in the story. I think it's really interesting to try and pursue. I tried it once, but the transitions were kinda a$$ with only changing when a different character on a different side was nearby.
What about you?
What do you think of no protagonists, only main characters & antagonists(+ villains)?
From a narrative sense, the Protagonist and Antagonist are two sides of the same coin, and both are fundamentally necessary to narrative storytelling, because it is through them that you, the author, are able to convey a narrative's conflict.
You can't actually have one, without the other, and they are a fundamental aspect of narrative storytelling.
The protagonist, is simply a primary character that undergoes a growth arc over the course of the narrative, while being the character followed by the narrative.
Now, if you mean "heroes" and "villains" instead, you can certainly write stories without any heroes or villains. The characters within the narrative may exist on different sides of a conflict (hence protagonist and antagonist) but are under no obligations to be heroic or villainous.
The protagonist is generally the perspective character the narrative (and reader) follow. They don't actually have to be heroic though. They can be anything you, the author, want them to be, while the antagonist is simply any character who opposes the protagonist.
So, let's say you have a narrative, and you are presenting it from the perspective of one character per chapter.
The perspective character is the protagonist of that chapter or segment. The characters who are opposed to them, are the antagonists of that chapter or segment.
But later, you switch to tell a later segment through one of the prior segment's antagonists. That character is now the protagonist of the new section, undergoing whatever story arc you have planned for them, while the characters who are opposing them (including your former protagonist) are now antagonists.
Hope this helps.